Microseismic Response of Rock Masses for Assessment of Caprock Integrity

• Author / Creator

  Khazaei, Cyrus

• Two major applications of microseismic (MS) monitoring in petroleum engineering are hydraulic fracturing operations and caprock integrity assurance. With respect to caprock integrity, this research has investigated the observation of the absence of detectable magnitudes of microseismic events as a postulate for no damage occurring within the caprock. The research explored the mechanisms of microseismic response of rock masses for the assessment and quantification of the extent of caprock damage occurring during geological storage of CO2 using continuum and discontinuum modeling approaches. In the first part of the thesis, microseismic response of rock masses is studied in two sections: intact rocks and weak planes. In order to study the microseismic response of intact rocks, a dataset of 73 uniaxial compression tests conducted by CANMET is analyzed. Acoustic emissions have also been recorded during the tests. Using the laboratory data, the relation between the total consumed energy and the released seismic energy for each type of rock is investigated. Using PFC3D, similar tests have also been conducted numerically. A modification of Gutenberg-Richter formula is proposed to get more realistic crack-induced magnitudes from PFC3D models. In order to quantify damage based on microseismic observations, a damage parameter is defined as the observed crack surface area during the test divided by the total possible crack surface area based on size of particles. The damage parameter is correlated with the cumulative released microseismic energy for each rock type at failure. In order to study the MS response of weak planes, a biaxial shear test on a specimen of Sierra granite with a single fault diagonally installed in it is numerically modeled using PFC3D. The details of experiment including microseismic data are extracted from literature. In order to get slip-induced microseismic data from PFC3D, a new algorithm is developed. The PFC3D results have been compared with real data. A parametric study is also conducted to investigate the effect of various parameters on stick-slip induced MS events. In the second part of the thesis, in order to explore the applicability of numerical modeling for assessment of caprock integrity using microseismic data, Weyburn CCS project has been studied as a case history. Therefore, a field-scale geomechanical model has been developed to study the likelihood of tensile and shear failure in the caprock of Weyburn. A one-way coupling has been conducted between the reservoir simulator (GEM) and the geomechanical simulator (FLAC3D). The results showed that in general, the likelihood of shear failure along preexisting weak planes has been about 27% greater than that of tensile failure. A discussion is also presented on the relevance of this study for interpretation of microseismic data recorded in caprock. Finally, in order to investigate the magnitudes of crack-induced and slip-induced MS emissions, a cubic representative elementary volume (REV) model with a weak plane at its center is made using PFC3D for the caprock of Weyburn reservoir. The REV is loaded to the conditions similar to those in the caprock of Weyburn. A parametric study has also been conducted to investigate the effect of varying factors on MS magnitudes. The results show that for the conditions studied in this research, slip-induced magnitudes range from ~-1 to -6 while crack-induced magnitudes range from ~-7 to -11. Therefore, considering the capability of geophones, crack-induced emissions may have been too small to be recordable in the caprock. Finally, a model is proposed to link the slip-induced microseismic energies to the state of stress in the surrounding of its parent weak plane. Considering the results of the coupled reservoir-geomechanical model as well as the magnitudes obtained from the REV model, it is concluded that microseismic monitoring in caprock is capable of recording slip-induced events greater than a certain magnitude. However, the lack of microseismic emissions recorded by geophones, does not rule out the possibility of crack development within intact parts as well as slow-slip deformations along weak planes. Also, microseismic monitoring does not seem to be capable of recording hydrofracture initiation in the caprock.

• Subjects / Keywords

  • Microseismic
  • Damage quantification
  • Weyburn
  • Stick-slip
  • Caprock integrity
  • Discrete element model (DEM)
  • Rockmass
  • Acoustic emission
  • Fault instability
  • Carbon capture and storage (CCS)
  • Geomechanics

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3JS9HJ9W

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.